1. Field of the Invention
The present invention relates to a digital communication, and in particular to a maximum likelihood symbol timing recovering unit for a symbol timing recovering process of a received signal.
2. Description of the Background Art
FIG. 1 is a block diagram illustrating a digital communication system of a known base bandwidth. As shown therein, the conventional digital communication system includes a transmitter 110 having a coder 111 and a transmission filter 122, a symbol decision unit 123, a decoder 124 and a timing recovering unit 125.
The operation of the conventional digital communication system will be explained.
When a binary bit BITS is inputted into the transmitter 110, the binary bit BITS is mapped in a certain alphabet by a coder 111 and is changed to a symbol am. The symbol am is converted into an analog signal s(t) by the transmission filter 112 having an impact response g(t) and is transmitted to the communication channel 130.
At this time, the communication channel 130 has a response characteristic b(t) in the case of a wired channel and becomes a receiving signal r(t) mixed with a summing noise n(t) by a thermal noise generated in the transmitter and receiver with respect to the signal s(t) transmitted to the receiver 120 by a wire or wireless method.
In addition, in the receiver 120 which receives the receiving signal r(t), the sampling of the symbol period is performed by the sampler 122 with respect to a signal which passes through the receiving filter 121 having the impact response f(t) like the impact response g(t) of the transmission filter 112 as a transmission characteristic.
Therefore, when the symbol decision unit 123 decides the At sampled data, the measured symbol value is obtained, and this symbol value is recovered into a binary bit by the decoder 124.
At this time, the sampler 122 samples an output signal of the receiving filter 121 by a sampling clock and transmits the data to the symbol decision unit 123. The sampling clock is generated by the timing recovering unit 125 which receives a receiving signal r(t).
The operation of the above-described circuit block will be explained with reference to a pulse amplitude modulation(PAM) of FIG. 2 and a frequency spectrum of the Nyquist pulse.
In the transmitter 110, the pulse g(t) having an amplitude decided based on the value of the transmission symbol am is generated at every symbol period T, and the thusly generated pulse g(t) is duplicated and becomes a transmission pulse s(t) which is a PAM signal as shown in FIG. 2A. The above-described operation may be implemented by the following Equation 1.
s(t)=xcexa3amg(txe2x88x92mT)xe2x80x83xe2x80x83Equation 1
The transmission pulse s(t) has a value xe2x80x9c0xe2x80x9d at all symbol points except for OTxe2x80x2.
The receiver 120 may recover the symbol value am at the transmission side in the case that the value is read at every symbol period nT by the sampler(or A/D converter) 122.
At this time, the operation for deciding an integrated impact response p(t) with respect to a response g(t) of the transmission filter 112, a response b(t) of the channel 130, and a response f(t) of the receiving filter 121 so that a zero cross point may occur at every symbol period nT except for OTxe2x80x2 is called as a Nyquist criterion. This response p(t) may be expressed in the following Equation 2.
p(t)=g(t)*b(t)*f(t)xe2x80x83xe2x80x83Equation 2
As shown in the spectrum of FIG. 2B, xe2x80x9cxcex1xe2x80x9d is a roll-off factor. When this value varies to 0xcx9c1, the over frequency bandwidth is changed to 0%xcx9c100%.
In addition, in the spectrum,   “            0      ~              1        2              ⁢    T    ”
bandwidth is called as a signal bandwidth, and the operation for carrying the transmission symbol am by multiple times is called as a Nyquist ratio transmission.
At this time, the frequency component at             1      2        ⁢    T    ,
namely, at the bandwidth edge point includes a timing information which is important for the symbol timing recovery. If the over frequency is decreased, the bandwidth occupying width of the channel is decreased, and it is difficult to obtain the timing recovery.
Therefore, in the digital communication, the output of the demodulator must be periodically sampled at the timing of tm=mT+xcfx84 based on the symbol rate.
Here, T represents a symbol interval, and xcfx84 represents a delay time which occurs during a transfer from the transmitter to the receiver.
In order to implement a periodical sampling operation, a clock signal is required for the receiver. The process for extracting the clock signal from the receiver is called as a symbol timing recovery.
Various methods for the symbol timing recovery are known.
First, in the spectrum recovering method(spectral line method), a band-pass filter is tuned at a bandwidth edge portion of the signal spectrum with respect to the receiving signal which passed through the linear or non-linear apparatus for thereby extracting a timing information.
For the binary signal, there are a method for checking a zero cross point and a method for using a point at which an inclination at the sampling time of the receiving signal becomes a timing information.
As an important factor for a selection of the timing recovering method, there are an area of the over frequency bandwidth and a level number of the signal. The case that random symbol value affects the timing information by a larger PAM signal which exceeds a certain signal level is called as a self-noise.
The circuit for the current timing recovery is directed to implementing a digital circuit.
Therefore, it is possible to enhance a reliability of the circuit operation by implementing a digital circuit for the sampling clock occurrence because that the signal process is digitally performed.
The digital implementation of the circuit for a timing recovery is obtained by a data interpolation method, a combination with a channel equalizer, etc. For example, a maximum likelihood symbol timing estimator is known.
The maximum likelihood (ML) symbol timing recovering unit uses a recovering technique for forming a likelihood function with respect to the receiving signal and estimating a timing phase for maximizing the likelihood function. The construction is different based on a DA(Data-Aided) ML, a DD(Decision-directed) ML, and a NDD (Non-Decision-directed) ML modes.
FIG. 3 is a block diagram illustrating a maximum likelihood symbol timing recovering unit using a DA(Data-Aided) ML mode as an example of the conventional art. As shown therein, the maximum likelihood symbol timing recovering unit includes a sampler 201 for sampling the matched and filtered signal at a certain period and outputting a digital signal qk{circumflex over ((xcfx84))} to a channel equalizer, a differential unit 202 for differentiating the matched and filtered signal, a sampler 203 for sampling the output signal of the differential unit 202 and outputting a digital signals             ⅆ              ⅆ        τ              ⁢          q      k        ⁢                  (        τ        )            ^        ,
a multiplier 204 for multiplying an accurate symbol am transmitted in a preamble format and an output signal       ⅆ          ⅆ      τ        ⁢      q    k    ⁢            (      τ      )        ^  
of the sampler 203, a k-term accumulator 205 for accumulating the outputs of the multiplier 204 at the m-symbol interval of the observing period, and a voltage adjusting oscillator VCO 206 for outputting an oscillation frequency to the samplers 201 and 203 using an output value of the accumulator 205 as an adjusting voltage.
The operation of the first example of the conventional art will be explained with reference to FIG. 3.
In the conventional art for the DA-ML mode, the matched and filtered signal is inputted into the sampler 201 and the differential unit 202, and an output of the differential unit 202 is inputted into the sampler 203. The-output of the sampler 203 and a symbol received in a preamble format are multiplied by the multiplier 204.
The resultant values accumulated from the multiplier 204 by the k-term accumulator 205 which performs a loop filter operation become a voltage which adjusts the voltage adjusting oscillator 206 for each k-symbol.
The output clock having its phase adjusted by the voltage adjusting oscillator 206 is inputted into the samplers 201 and 203 as a clock signal for thereby estimating the operation timing phase.
The symbol ak inputted into the multiplier 204 is a training sequence which is periodically inputted in the information symbol.
The above-described operation will be explained with reference to FIG. 1.
The base bandwidth PAM signal s(t) outputted from the transmitter 110 is expressed as a function of the phase xcfx84 based on the following Equation 3.                               s          ⁡                      (                          t              ;              τ                        )                          =                              ∑                          k              =              ∞                        ∞                    ⁢                      xe2x80x83                    ⁢                                    a              k                        ⁢                          g              ⁡                              (                                  t                  -                  kT                  -                  τ                                )                                                                        Equation        ⁢                  xe2x80x83                ⁢        3            
where ak represent a transmission symbol without a noise, and g(t) represents a Nyquist pulse.
At this time, as shown in Equation 3, when a signal s(t) is transmitted to the communication channel 130, a matched filtered signal r(t) which is added with an additive white gaussian noise (AWGN) and is inputted into the samplers 122 and 201 may be expressed as Equation 4, and a log likelihood function may be expressed as Equation 5.
r(t)=s(t;xcfx84)+n(t)xe2x80x83xe2x80x83Equation 4
                                          Λ            L                    ⁢                                    (              τ              )                        ^                          =                              1            N0                    ⁢                                    ∑              k                        ⁢                                          a                k                            ⁢                                                q                  k                                ⁡                                  (                  τ                  )                                                                                        Equation        ⁢                  xe2x80x83                ⁢        5            
In the sampler 201, the digital signal qk{circumflex over ((xcfx84))} may be expressed as Equation 6.
qk{circumflex over ((xcfx84))}=∫Tor(t)g(txe2x88x92tKxe2x88x92xcfx84)dtxe2x80x83xe2x80x83Equation 6
In the above Equation 6, xcfx84 represents a phase estimated by the receiver 120.
Therefore, the value sampled by the phase xcfx84 of the rise integration phase of the receiving signals r(t) and g(t) becomes qk(xcfx84) of Equation 6.
Here, the configuration that the response f(t) of the receiving filter 121 becomes identical with the response g(t) of the transmission filter 112 in their characteristics is called as a matched filtering, and the integration interval TO=kT represents an observing interval in Equation 6.
At this time, the necessary conditions for maximizing the likelihood function xcex9L{circumflex over ((xcfx84))} as shown in Equation 5 with respect to the phase {circumflex over (xcfx84)} is implemented based on the following Equation 7.
This likelihood function is generated by the k-term accumulator 205.                                                         Λ              L                        ⁡                          (              τ              )                                            d            ⁢                          xe2x80x83                        ⁢            τ                          =                              ∑            k                    ⁢                                    a              k                        ⁢                                          ⅆ                                  xe2x80x83                                                            ⅆ                τ                                      ⁢                          q              k                        ⁢                                          (                τ                )                            ^                                                          Equation        ⁢                  xe2x80x83                ⁢        7            
As seen in Equation 7, the loop is formed so that the value       ⅆ          ⅆ      τ        ⁢      q    k    ⁢            (      τ      )        ^  
sampled at a period T by the sampler 203 with respect to the output value of the differential unit 202 which differentiates the matched filtered signal, is multiplied with the symbol am transmitted in a preamble format by the multiplier 204, and a resultant value of the multiplication is outputted, and the m-term accumulator 206 accumulates the resultant values for the k-symbol of the observing interval and outputs the accumulated values as an adjusting voltage of the voltage adjusting oscillator 206 based on the adjusting voltage 0.
Namely, the k-term accumulator 205 which acts as a loop filter accumulates the output signal of the multiplier 204, and a result of the accumulation becomes an adjusting voltage of the voltage adjusting oscillator 206 for each k-symbol.
The case that the transmission symbol value at the side of the receiver 120 is involved in the training mode. This method is called as a DA-ML mode(Data-Aided ML mode).
On the contrary, a certain operation is needed to obtain an average of the likelihood function xcex9L{circumflex over ((xcfx84))} of Equation 7 based on the probability of the symbol ak not to use an accurate symbol value ak received in a preamble format.
At this time, the average /xcex9L{circumflex over ((xcfx84))} of the log likelihood function xcex9L{circumflex over ((xcfx84))} using the probability of the symbol ak based on the gaussian distribution may be expressed as Equation 8.                               /                                    Λ              L                        ⁡                          (              τ              )                                      =                              1                          2              ⁢              N0                                ⁢                                    ∑                              k                =                ∞                            ∞                        ⁢                          xe2x80x83                        ⁢                                          q                k                2                            ⁢                                                (                  τ                  )                                ^                                                                        Equation        ⁢                  xe2x80x83                ⁢        8            
In order to search the phase xcfx84 for maximizing the value of Equation 8, a certain adjusting voltage is obtained for the voltage adjusting oscillator 206 so that the differentiated value of Equation 8 becomes 0 with respect to the phase xcfx84.                                                         ⅆ                              xe2x80x83                                                    ⅆ              τ                                ⁢                                    ∑              k                        ⁢                                          q                k                2                            ⁢                                                (                  τ                  )                                ^                                                    =                              2            ⁢                                          ∑                k                            ⁢                                                                    q                    k                                    ⁡                                      (                    τ                    )                                                  ⁢                                                                            ⅆ                      qk                                        ⁢                                                                  (                        τ                        )                                            ^                                                                            ⅆ                    τ                                                                                =          0                                    Equation        ⁢                  xe2x80x83                ⁢        9            
This method is called as a NDA-ML(Non-Data aided ML).
The NDA-ML mode may be classified into a DD-ML or NDD-ML mode, and the circuit may be constructed based on each mode as shown in FIGS. 4 and 5.
FIG. 4 is a block diagram illustrating a maximum likelihood symbol timing recovering unit using a DD-ML mode according to a second example of the conventional art. As shown therein, there are provided samplers 201 and 203, a differential unit 202, a k-term accumulator 205, a voltage adjusting oscillator 206, a symbol value estimator 212 for estimating a symbol ak based on a digital signal qk{circumflex over ((xcfx84))} from the sampler 201, and a multiplier 211 for multiplying the symbol from the symbol value estimator 212 and the output signal       ⅆ          ⅆ      τ        ⁢      q    k    ⁢            (      τ      )        ^  
of the sampler 203 and outputting to the k-term accumulator 205.
FIG. 5 is a block diagram illustrating a maximum likelihood symbol timing recovering unit using a NDD-ML mode according to a third example of the conventional art. As shown therein, there are provided samplers 201 and 203, a differential unit 202, a k-term accumulator, a voltage adjusting oscillator 206, and a multiplier 213 for multiplying a digital signal qk{circumflex over ((xcfx84))} from the sampler 201 and an output signal       ⅆ          ⅆ      τ        ⁢      q    k    ⁢            (      τ      )        ^  
from the sampler 203.
As shown in FIGS. 4 and 5, the operations of the second and third examples are different in their multiplications. The different operation therebetween will be explained.
When the sampler 201 samples the matched filtered signal and outputs the digital signal qk{circumflex over ((xcfx84))} to the channel equalizing unit, in the second example of FIG. 4, the symbol value estimator 212 receives the digital signal qk{circumflex over ((xcfx84))} and estimates a symbol ak and inputs the same into the multiplier 211, and in the third example of FIG. 5, the digital signal qk{circumflex over ((xcfx84))} is directly inputted into the multiplier 213.
At this time, the multipliers 211 and 213 multiply the digital signal       ⅆ          ⅆ      τ        ⁢      q    k    ⁢            (      τ      )        ^  
from the sampler 203 and the output signal from the sampler 201 and outputs a result of the multiplication to the k-term accumulator 205.
Therefore, the k-term accumulator 205 which performs a loop filter operation accumulates an output signal from the multiplier 204 and outputs the same as an adjusting voltage of the voltage adjusting oscillator 206 at each k-symbol, so that an output clock of the voltage adjusting oscillator 206 in which the phase xcfx84 is adjusted is inputted as the clock signals of the samplers 201 and 203 for thereby optimally estimating the operation timing phase.
In the conventional art, the differential operation is performed with respect to the matched filtered signal which is in an analog signal state before the sampling is performed. The differential unit 202 has a group delay and generates an off-set of a sampling clock signal when the signal is not delayed at a previous circuit of the sampler 201 as long as a certain delay time.
Therefore, in the conventional art, if the sampling position is moved toward a certain position at a distance from the accurate position, an error may be increased as a result of the decision which is made using the sampled value.
In addition, in the conventional art, the analog circuit (differential unit 202) and the digital circuit are co-used, so that a signal processing process is complicated and a signal processing speed is decreased.
Accordingly, it is an object of the present invention to provide a maximum likelihood timing recovering unit which is capable of recovering an optimum timing using a data over-sampled by adjusting an oscillation speed of a voltage adjusting oscillator by two times.
In the present invention, it is possible to remove a conventional complicated signal process by removing a timing off-set which occurs when obtaining a decision strobe sample by providing a delay unit having a delay value which is the same as a group delay amount of an analog differential unit, removing one sampler and substituting a conventional analog differential unit with a digital differential unit.
In the DD-ML mode and SD(sign Decision) 0ML mode, the oscillation speed of the voltage adjusting oscillator is controlled to be same as the symbol ratio sampling speed for thereby recovering an optimum timing.
In a maximum likelihood symbol timing recovering unit, an approximate inclination is obtained at a sampling point using two delay units, and a decision sample(strobe) is multiplied with the inclination or a symbol decision value or a code of a decision sample.
In a digital communication system which has a symbol timing recovering unit for a recovering operation by periodically sampling a receiving signal according to the present invention, the symbol timing recovering unit includes a sampler for over-sampling a matched filtering signal, first and second delay units for sequentially delaying a sampling signal of the sampler, a first decimeter for outputting a decision sample among an output signal of the first delay unit to a symbol decision unit, an adder for adding an output of the sampler and an output of the second delay unit, a multiplier for multiplying a decision symbol ak received in a preamble format and an output signal of the adder, a second decimeter for outputting a certain interval value based on a decision sample position among the output of the multiplier, an accumulator for accumulating the output of the second decimeter at a certain interval, and a voltage adjusting oscillator for providing an oscillation frequency generated using the output of the accumulator as an adjusting voltage to a 2Fs sampling clock signal of the sampler.
The accumulator may be substituted with a multiplier which is capable of multiplying a certain sept coefficient with an output of a second decimeter and outputting a result of the multiplication to a voltage adjusting oscillator.
In addition, in the case of the SA-ML mode and DD-ML mode, two decimeters may be removed by implementing a symbol ratio sampling speed.
Additional advantages, objects and features of the invention will become more apparent from the description which follows.